docs: def radius(self, angle):
          """
          Calculate the polar radius for a given polar angle.
  
          Parameters
          ----------
          angle : float
              The polar angle (radians).
  
          Returns
          -------
          radius : float
              The polar radius (pixels).
          """
  
          return (self.sma * (1. - self.eps) /
                  np.sqrt(((1. - self.eps) * np.cos(angle))**2 +
                          (np.sin(angle))**2))